On Simulating Ultrafast Chemical Processes and their Spectroscopic Signatures
To improve technologies for, e.g., solar cells, we must understand how molecules are affected by absorption of UV and visible light. Thus, it must be investigated what happens upon absorption as well as over time. Here, the movement of molecules must be taken into account. Experimentally, chemical reactions are followed by imaging the molecule at different time delays, which creates a sort of movie. To do this, one can, e.g., use X-rays, which allow specific atoms to be singled out. Experiments, however, are often difficult to interpret and require a theoretical understanding. In this thesis it has been investigated how to computationally describe such movies based on X-ray spectroscopy and particularly X-ray absorption spectroscopy (XAS).
A generally very accurate method for describing the individual frames of the movies has been investigated for X-ray spectroscopies, and it was found to describe these well. Moreover, another popular and less demanding method was considered for XAS, and it turned out that its description here was on equal footing with the more demanding method. For large systems it might therefore be a good alternative.
Molecular movements and populations of electronic states were investigated by describing the nuclei using classical mechanics. This, overall, yields good results and can identify the most important degrees of freedom (DOFs) for the movements. Therefore, such a calculation can also be used as the basis for a quantum mechanical one, which is only possible for a limited number of DOFs.
For each electronic state and molecular structure an XAS frame can be determined. The total frame for the movie at a given time can then be made as a weighted sum of these XAS frames based on the populations of the electronic states. Hence, by combining the above mentioned calculations, a movie of a molecular reaction can be obtained. It turns out that the XAS frames hardly change for small molecular movements, while the electronic state has a great effect. Thus, all frames can be determined based on the same molecular structure, which saves time and computational resources. If the molecule stays in the same electronic state for a long time, it might even be possible to avoid determining the state populations. More investigations are needed to determine if this protocol can be extended to, e.g., non-rigid molecules.
The PhD defence will be broadcasted at:
https://dtudk.zoom.us/j/62298491050?pwd=ZkdjYUhzc3NtcUVuSnVucnVnU2puZz09
Principal Supervisor:
Professor Klaus Braagaard Møller, DTU Chemistry
Co-supervisor:
Professor Sonia Coriani, DTU Chemistry
Professor Henrik Koch, NTNU, Norway
Examiners:
Associate Professor Niels Engholm Henriksen , DTU Chemistry
Professor Christof Hättig , Ruhr Universität Bochum, Germany
Professor Nađa Došlić , Ruđer Bošković Institute , Zagreb, Croatia
Chairperson:
Professor Martin Meedom Nielsen, DTU Physics